European Patent 70,540, and West German Patent Application Nos. 3,121,289 and 3,121,290 describe resistors (hereinafter referred to as "varistors") obtained by increasing the resistance at grain boundaries of strontium titanate-base semiconductor ceramics. These varistors are used as ring varistors for absorbing sparks in micromotors. Ring varistors must have a high flexural strength because they are usually fitted in position without a resin dip. This high flexural strength is essential, particularly for varistors for micromotors because of their small size.
Varistors made of the conventional strontium titanate-base semiconductor ceramic described in the above references are low in flexural strength. In order to provide a flexural strength sufficient for practical use, therefore, it is necessary to increase the thickness of varistors to about 1 mm. Such varistors of increased thickness are not suitable for use in micromotors which are desired to be miniaturized.